It's actually one of the most effective sales strategies for dropshipping, bulk listings. Not advisable for anyone just starting out, but once you get enough market research behind you and you have a good idea on what sells and for how much and when, then a bulk listing strategy can be a very effective one.
Down side, labor intensive and hi fees due the the listing volume. So you need to have an idea of what you're doing before you try that sort of thing and it's also advisable to have a shop as well to keep the listing fees down as much as possible.
So the chances are, anyone with a massive stock profile is probably an experienced dropshipper or even a wholesaler using eBay as their own marketplace to move product.